The new car market has declined for a sixth consecutive month, industry figures show.
Just over 426,000 new cars were registered in September, down 9.3% on the same month last year, according to the Society of Motor Manufacturers and Traders (SMMT).
The organisation blamed a fall in consumer confidence caused by economic and political uncertainty, and confusion over air quality plans.
Some 2.07 million new cars have been registered so far this year, a decrease of 3.9% on the same period in 2016.
SMMT chief executive Mike Hawes said: "September is always a barometer of the health of the UK new car market so this decline will cause considerable concern. Business and political uncertainty is reducing buyer confidence, with consumers and businesses more likely to delay big-ticket purchases.
"The confusion surrounding air quality plans has not helped, but consumers should be reassured that all the new diesel and petrol models on the market will not face any bans or additional charges.
"Manufacturers' scrappage schemes are proving popular and such schemes are to be encouraged given fleet renewal is the best way to address environmental issues in our towns and cities."